Most good dog foods print their energy value on the bag, the can or the maker’s website. Some do not, and in a few countries labels are not required to. When energy is missing, you can still estimate it from the guaranteed analysis, using factors that nutritionists have standardised. The estimate is rougher than the maker’s own figure, but it is far better than guessing, and it lets you compare foods and work out grams. This guide shows the method step by step, with worked examples for a dry and a wet food, explains the limits and shows how to use the result. The Dog Food kcal Calculator does the arithmetic with the same factors.
The factors
The NRC (2006) publishes modified Atwater factors for pet foods, which estimate metabolisable energy from the main nutrients:
| Nutrient | kcal per gram |
|---|---|
| Crude protein | 3.5 |
| Crude fat | 8.5 |
| Nitrogen-free extract (carbohydrate) | 3.5 |
These are lower than the “general” Atwater factors of 4, 9 and 4, because they allow for the incomplete digestion of pet food. Fibre is not counted.
Step 1: collect the label numbers
From the guaranteed analysis or “analytical constituents”, write down crude protein, crude fat, crude fibre and moisture, as percentages. Ash is often missing. If so, assume a typical value: about 8% for dry food and 2.5% for wet, which are the defaults used in our calculator. If the label gives maximums for fibre and moisture and minimums for protein and fat, use the figures as given; the result is approximate.
Step 2: find NFE by difference
The label does not give carbohydrate. Estimate it by subtraction:
NFE % = 100 − protein − fat − fibre − ash − moisture
Step 3: apply the factors
kcal per 100 g = 3.5 × protein + 8.5 × fat + 3.5 × NFE
Multiply by 10 for kcal per kg.
Worked example 1: a dry food
Label: protein 26%, fat 14%, fibre 4%, moisture 10%, ash 7%.
| Step | Working | Result |
|---|---|---|
| NFE | 100 − 26 − 14 − 4 − 7 − 10 | 39% |
| Protein energy | 26 × 3.5 | 91 |
| Fat energy | 14 × 8.5 | 119 |
| NFE energy | 39 × 3.5 | 136.5 |
| kcal per 100 g | 91 + 119 + 136.5 | 346.5 |
| kcal per kg | × 10 | 3,465 |
A neutered 20 kg dog needing 1,059 kcal would eat 1,059 ÷ 3.465 = 306 g of this food.
Worked example 2: a wet food
Label: protein 8%, fat 5%, fibre 1%, moisture 78%, ash 2%.
| Step | Working | Result |
|---|---|---|
| NFE | 100 − 8 − 5 − 1 − 2 − 78 | 6% |
| Protein energy | 8 × 3.5 | 28 |
| Fat energy | 5 × 8.5 | 42.5 |
| NFE energy | 6 × 3.5 | 21 |
| kcal per 100 g | 28 + 42.5 + 21 | 91.5 |
| kcal per kg | × 10 | 915 |
A 400 g can of this food has about 366 kcal. The same dog would need 1,059 ÷ 0.915 = 1,157 g of this food a day if it were the only food. Compare with the dry food: 306 g. The water explains the difference; see dry matter basis explained.
How accurate is it?
The estimate is useful for comparing foods and getting started, but it is not exact. Reasons:
- The label gives minimum or maximum values, not the actual analysis.
- Ash is often assumed.
- Fibre and digestibility vary, and the factors are averages.
- Foods with high fibre or unusual ingredients may deviate.
- Rounding adds small errors.
As a rough guide, estimates from labels are often within 10 to 15% of the maker’s metabolisable energy figure, but they can be farther out. If the maker’s energy value is available, use it. If the two disagree widely, ask the maker which is right.
Using the result
- Take your dog’s daily calories from the Dog Food Calculator.
- Divide by the estimated kcal per gram.
- Feed that amount for two to four weeks.
- Weigh the dog and score body condition.
- Adjust by about 10% in the right direction.
Because the estimate is rough, steps 3 to 5 are what make the amount right for your dog. Our guide to reading a dog food label shows where each number sits on the bag.
Estimating for cats
The same factors are used for cat foods, and the method is identical; use the cat’s analysis. Cat foods are usually higher in protein and fat, so the energy per kg tends to be higher for the same moisture. The Pet Food Energy Converter switches between kcal per kg, per 100 g and per cup once you have the estimate.
Comparing two foods with no energy on the label
Run both through the method, then compare cost per 1,000 kcal. Suppose food X has 3,465 kcal/kg at 3.80 per kg and food Y has 3,900 kcal/kg at 4.60 per kg.
| Food | kcal per kg | Price per kg | Cost per 1,000 kcal |
|---|---|---|---|
| X | 3,465 | 3.80 | 1.10 |
| Y | 3,900 | 4.60 | 1.18 |
Y costs 7% more per 1,000 kcal. Whether that is worth it depends on ingredients, suitability and your vet’s advice. The Pet Food Price per 1,000 kcal Calculator does the comparison.
Common mistakes
- Forgetting to subtract ash and moisture when finding NFE.
- Using dry matter percentages with as-fed factors.
- Treating the estimate as exact.
- Using general Atwater factors (4, 9, 4) instead of the modified ones.
- Ignoring fibre.
- Using a maximum moisture as the actual.
- Not checking the dog’s weight afterwards.
Questions to ask the maker
- What is the metabolisable energy in kcal per kg?
- How was it determined, by feeding trial or calculation?
- What is the typical ash content?
- Has the formula changed?
- What is the batch variation?
Worked example 3: a food with high fibre
Label: protein 22%, fat 9%, fibre 9%, moisture 10%, ash 8%.
| Step | Working | Result |
|---|---|---|
| NFE | 100 − 22 − 9 − 9 − 8 − 10 | 42% |
| Protein energy | 22 × 3.5 | 77 |
| Fat energy | 9 × 8.5 | 76.5 |
| NFE energy | 42 × 3.5 | 147 |
| kcal per 100 g | 300.5 | |
| kcal per kg | 3,005 |
This lower-energy food, typical of some weight-management foods, would need 1,059 ÷ 3.005 = 352 g a day for a neutered 20 kg dog, compared with 306 g for the first example. Because fibre dilutes the energy, the dog eats more grams for the same calories. The method captures this automatically, as fibre takes up share of the 100% without adding energy.
Checking your estimate against the maker’s figure
If the maker also gives metabolisable energy, compare. Suppose your estimate for the first food is 3,465 kcal/kg and the maker says 3,520. The difference is 55 kcal/kg, or 1.6%, which is within the expected margin. If the maker says 3,900, the difference is 12%, and you should wonder whether the label’s numbers are minimums or the maker uses a more sophisticated equation. Use the maker’s value for feeding, but keep your estimate as a sanity check.
When the label gives dry matter values
A few labels, and many nutrition tables, give values on a dry matter basis. To estimate energy from dry matter values, convert to as-fed first by multiplying by (100 − moisture) ÷ 100, then use the method. For example, protein 29% on dry matter and moisture 10% gives 26.1% as fed. See dry matter basis explained for the conversions. Mixing the two bases is the most common source of wrong answers.
Questions owners ask in more depth
The maker says 3,800 kcal/kg but my calculation says 3,500. Who is right? The maker’s figure is usually derived from feeding trials or a detailed equation and is more reliable. Your estimate is a check.
Do I use “crude” values straight? Yes; they are what the label gives.
What if moisture is missing? Assume about 10% for dry and 78% for wet, or ask the maker. A wrong moisture figure shifts NFE and the result.
Does the method work for raw or fresh food? Yes, if you have the analysis, though digestibility differs. Ask a veterinary nutritionist.
Can I use it for treats? Yes, with caution; treats can have unusual ingredients.
What is the difference between gross and metabolisable energy? Gross energy is the energy in the food, and metabolisable energy is what is left after losses in faeces and urine; labels should show metabolisable energy.
Calories in a whole bag
Once you have kcal per kg, you can compute the energy in a bag and plan how many days it feeds. A 10 kg bag of the first example food (3,465 kcal/kg) holds 34,650 kcal. A neutered 20 kg dog needing 1,059 kcal a day gets 32.7 days from it. Compare with the days from grams: 10,000 ÷ 306 = 32.7. The same answer arises either way, which is a useful check on your arithmetic. For several pets, add their daily calories. Our guide to how long a bag of dog food lasts shows the grams route.
Estimating from a partial label
Some labels list only protein, fat and fibre and moisture. If ash is missing, assume the typical value and note that you did. If moisture is missing, ask the maker. If the label shows only “protein min 25%, fat min 12%”, you can still estimate, but with more uncertainty; use a range by trying both the minimum and a plausible typical value. A rough range is more honest than a single number. Keep a record of your assumptions with the result, so that you can revise them if the maker supplies the real figures.
A caution about prescription and raw foods
The estimate is designed for typical commercial foods. Prescription diets, raw foods, freeze-dried products and foods with unusual ingredients can have digestibility that differs from the average. For these, use the energy value given by the maker or your vet, and do not rely on an estimate. If no value is available, ask for one. Veterinary nutritionists can calculate energy for home-prepared diets with software that uses the ingredient values rather than label percentages.
Quick summary
- Estimate energy with modified Atwater factors: protein 3.5, fat 8.5 and NFE 3.5 kcal per gram.
- NFE = 100 − protein − fat − fibre − ash − moisture.
- Example dry food: 3,465 kcal/kg; example wet food: 915 kcal/kg.
- Treat the result as an estimate within perhaps 10 to 15%.
- Feed, weigh and adjust by about 10%.
- Use the Dog Food kcal Calculator.
Frequently asked questions
How can I work out calories if the bag does not show them?
Use modified Atwater factors on the guaranteed analysis: protein 3.5, fat 8.5 and nitrogen-free extract 3.5 kcal per gram. Or ask the maker.
What is NFE?
Nitrogen-free extract, an estimate of digestible carbohydrate found by subtracting protein, fat, fibre, ash and moisture from 100.
How accurate is the estimate?
It is rough: perhaps within 10 to 15% of the maker's figure. Use the maker's metabolisable energy when available.
Which numbers do I need from the label?
Crude protein, fat, fibre, moisture and, if shown, ash. If ash is missing, a default is assumed.
Does the estimate apply to wet food?
Yes, with the same factors. Wet food has little energy per gram because of the water.
Why do makers' figures differ from the estimate?
Makers may use feeding trials or more detailed equations, which reflect digestibility.
What do I do with the result?
Divide your dog's calories by the estimate to get grams, then adjust from weight.
Sources
- NRC (2006) Nutrient Requirements of Dogs and Cats, National Academies Press: modified Atwater factors
- WSAVA Global Nutrition Guidelines
Related calculators
- Dog Food Calorie CalculatorWork out how many calories are in a dog or cat food from the nutrient percentages (guaranteed analysis) on its label, when no kcal figure is printed.
- Dry Matter Basis CalculatorCompare wet and dry pet foods fairly by removing the water from the label percentages.
- Pet Food Energy ConverterConvert pet food energy between kcal per kg, per 100 g, per pound and per ounce.
Keep reading
- How to Read a Dog Food Label: kcal and AnalysisHow to read a dog food label: the adequacy statement, kcal per kg or cup, guaranteed analysis versus dry matter, ingredient order and a step-by-step comparison.
- Dry Matter Basis: Compare Wet and Dry FoodLearn how dry matter basis works, the formula to convert any pet food label, worked examples for wet, dry and semi-moist foods and common label traps.
- Why the Feeding Guide on Your Dog Food Bag Is OffWhy a dog food bag's feeding guide often overshoots: wide weight bands, no allowance for neutering or treats, and how to fix it with a calorie calculation.
- When Ash Is Missing on a Pet Food LabelEstimate calories when ash is not listed: use 8% for dry and 2.5% for wet food. A 4-point ash swing moves a dry food about 4% (336 to 350 kcal per 100 g).